Heating element

ABSTRACT

The heating element is formed of refractory material resistant to oxidation, in the general shape of a parallelepiped. It has, in a direction perpendicular to two opposite surfaces, a nonhomogeneous structure such that when one of the two surfaces concerned is brought to a sufficiently high temperature to become electrically conducting, the other remains at a sufficiently low temperature to be thermally and electrically practically nonconducting. The structure is adapted to withstand movements due to differences in expansion. The element is made either of entirely or partially stabilized zirconia or of a definite compound of the pyrochlore type based on zirconia, of the formula Zr2T2O5, T being a metal, especially a rare earth. It has holes and cut-outs formed in the mass of the element in a direction substantially perpendicular to that which unites said two opposite surfaces. The ratio hollow/solid is within a range of about 0.1 to 3.

United States Patent [191 Anthony et a1.

[ Feb. 12, 1974 HEATING ELEMENT [73] Assignee: Agence Nationale De Valorisation De La Recherche (AI-IVAR), Paris, France [22] Filed: May 17, 1972 [21] Appl No.: 254,034

[30] Foreign Application Priority Data May 19, 1971 France .1 71.18236 [52] U.S. Cl 219/553, 338/322, 338/333, 338/334 [51] Int. Cl. H05b 3/10 [58] Field of Search 338/316, 333, 334, 322; 13/22, 1 3/25, 35; 219/553 OTHER PUBLICATIONS New Zirconia Materia in a New Material Preview, March 1951, p. 81

Primary ExaminerE. A.'Goldberg Attorney, Agent, or Firm--Larson, Taylor & Hinds [5 7] ABSTRACT The heating element is formed of refractory material resistant to oxidation, in the general shape of a parallelepiped. It has, in a direction perpendicular to two opposite surfaces, a nonhomogeneous structure such that when one of the two surfaces concerned is brought to a sufficiently high temperature to become electrically conducting, the other remains at a sufficiently low temperature to be thermally and electrically practically non-conducting. The structure is adapted to withstand movements due to differences in expansion. The element is made either of entirely or partially stabilized zirconia or of a definite compound of the pyrochlore type based on zirconia, of the formula Zr T O T being a metal, especially a rare earth. It has holes and cut-outs formed in the mass of the element in a direction substantially perpendicular to that which unites said two opposite surfaces. The ratio hollow/solid is within a range of about 0.1 to 3.

14 Claims, 18 Drawing Figures PATENTEB FEB I 2 I974 Sam 1 or 3 PATENTEUFEB 1 2 m4 SHEET 2 [IF 3 1 HEATING ELEMENT The invention relates to a heating element of the type formed of a refractory material resistant to oxidation, with melting point above 2,500C, and generally known as resistors.

In present heating elements of the type concerned -which bound individually a heating cavity by their shape or which are assembled to constitute heating cavities of any shape and size, it is necessary to provide bulky and sometimes heavy heat insulating means.

It is a particular object of the invention to overcome this drawback and to place at the disposal of the user a heating element of the type concerned which responds to the various desiderata of pracice better than those existing hitherto.

The heating element according to the invention is characterised by the fact that it is of general shape close to that of a parallelopiped and that it has, in a direction perpendicular to two opposite surfaces, a nonhomogeneous structure such that when one of the two surfaces concerned is brought to a sufficiently high temperature to become electrically conducting, the otherremains at a sufficiently low temperature to be practically non-conductive of electricity, the said structure being adapted to support the movements due to differences of expansion.

The invention consists, apart from the above-said features, of certain other features which are used, preferably at the same time and which will be more explicitly considered below.

And it will in any case be well understood with the aid of the additional description which follows, as well as of the accompanying drawings given of course purely by way of illustrative but non-limiting example.

In the drawings:

FIGS. 1, 2 and 3 show respectively in perspective for the first and in elevation for the two others, three modifications of the first embodiment of the heating element according to the invention.

FIGS. 2a and 3a are views of FIGS. 2 and 3 along the lines 2a 2a and 3a 3a.

FIGS. 4, and 6 show in elevation three modifications of a second embodiment of a heating element according to the invention.

FIGS. 4a, 5a and 6a are views along the lines 4a--4a, 5a5a and 6a-6a of FIGS. 4 to 6.

FIGS. 7 to 10 show in perspective assemblies of several heating elements according to the invention.

FIG. 11 shows in elevation a fourth modification of the above-said first embodiment.

FIGS. 12 and 13, lastly, show respectively in elevation and in view and partial section along the lines XIII- XIII of FIG. 12, a fourth modification of the abovesaid first embodiment.

According to the invention and, more especially, according to those types of its application, as well as according to those embodiments of its various parts, to which it would appear that preference should be given, in order to construct a heating element of the type concerned, procedure is as follows or in similar manner.

The general shape of the said heating element, which is made of a refractory material resistant to oxidation of the type used for the manufacture of resistors, is parallel-epipedic or close to that of a parallelopiped. It has, consequently, the general shape of a brick of which the length L is greater than the height H, itself greater than the thickness E.

'Between two opposite surfaces denoted by F, and F (in general those which are separated by a distance equal to the height H of the element) and in a direction substantially perpendicular to these two surfaces, the structure of said element is not homogeneous so that when one of the two faces F, and F namely face F,, is brought to a sufficient temperature to become electrically conducting, the other remains sufficiently cool to remain practically non-conducting, by means of which its face F and the constituant material comprised between the faces F, and F,, the heating element constitutes its own heat insulation. In addition, this construction is such that it withstands movements due to differences in expansion.

In practice, it is arranged that the difference in temperature between the two faces F, and F is such that the resistance of thecool face is at least times higher than the resistance of the hot face.

The supply of electric current to the heating element is effected at the level of the two ends of the element taken in the sense of the length, for example by means of platinum contacts.

In general, the constituant material of the elements according to the invention is partially or entirely stabi lised zirconia in its form stable at hightemperature that is to say its cubic form. 7

It is known that this stabilisation is obtained by the addition of lime yttrium oxide, rare earth oxides, magnesia or the like.

The elements according to the invention can also be constituted of a definite compound of the pyrochlore type based on zirconia, of the formula Zr T O T being a-metal, preferably a rare earth;

The above-said non-homogeneity of structure can be effected in different ways.

In a first embodiment, this absence 'of homogeneity, due to which the transmission of heat from face 1 to face 2 is sufficiently limited for the face F to remain at a temperature at which it is not conducting, is obtained by holes and cut-outs formed in the mass of the heating element in the direction substantially perpendicular to that which connects the faces F, and F, the ratio cavity/solid, when the constituent material of the element is partially or entirely stabilised zirconia, being selected around 0.l-0.2 to 3.

The arrangement and configuration of the above-said holes and cut-outs are selected so that when the constituent material of the element which is situated in the vicinity of the face F, is brought to the temperature from which it becomes conducting, the electric current only passes in fact in the vicinity of the said face F,.

In FIGS. 1, 2 and 3, there are shown three modifications of this first embodiment.

In the case of the modification of FIG. 1, the heating element, denoted as a whole by 1, comprises a plurality of the holes 2 as shown, as well as cut-outs 3 in the form of a slot along a broken line comprising parts parallel to a certain length, at 30, to the face F,, as well as parts 3band 3c of a general shape substantially perpendicu lar to the face F,, the assembly being such that the said face F, is connected to the cold face F, by parts 4 of the element which, as a result of the selection of the cutouts 3, are of relatively slight cross-section. Due to this constitution, the transmission of heat from the face F, to the face F is very much reduced.

In addition, the positioning of the parts 3a is selected so that the thickness of the element at the level of the face F,, through which thickness the electric current passes, is relatively slight. In practice, this thickness, denoted by D, in the Figure, as well as that of the parts 4 denoted by D is of the order of 1-3 to 5-10 mm.

It is advantageous to provide at the ends (taken in the sense of the length, of the element) holes of large size 5 of which the shape can be that which is apparent from FIG. 1 and which can serve, on one hand, for the suspension of the element, and, on the other hand, for the localisation of the leads for the introduction of the current 6, which are provided advantageously, as seen in the Figure, at the level of that of the edges of the holes 5 which occurs nearest the face F,. These current leads can be constituted by platinum plates bounded under pressure or moulded.

It may be advantageous to arrange the heating element 1 so that the temperature reached by the face F, is progressive over at least a part of its length. Thus, for example, it is desired in the case of the modification of FIG. 1 that the progression occurs in the direction increasing along the arrow F,.

To obtain this result, it suffices to select the size and the arrangement of the said holes and cut-outs so that there is a progressive increase in the resistance at the level of the face F, in the direction of the arrow F,. For example, as shown in FIG. 1, there may be conferred on the first cut-out 3 starting from the left a part 3d situated between the part 3b and the part 3a, and included so that the cross-section of the part rendered conducting on operating the element diminishes as shown.

To avoid deterioration and placing out of service of an element 1 as a result of the formation of a crack interrupting the electric current and which could start at the level of face F,, it is advantageous to divide as shown in FIG. 1 the face F, into several parallel lamellae 7a, by means of cut-outs 7b, perpendicular to the face F,.

This being the case, it is indicated that in the case of the modifications'of FIGS. 2 and 3, the same principles as those described with regard to FIG. 1 have prevailed, although the holes and cut-outs adopted are of different shapes.

In fact, it is with the rectangles 8 and the cut-outs 9 of U or I shape in the case of FIG. 2, with circular holes 10 in the case of FIG. 3, that corresponding elements have been arranged so that the faces F, and F are only connected by relatively narrow bridges P, thus sufficiently reducing the heat transmission from the face F, to the face F,. In addition, the said holes and cut-outs are selected so that in the case of FIGS. 2 and 3 re course is had especially to an elongated inclined cutout 11 of which the arrangements and size are apparent from the said Figures the electric current can only pass through the face F,.

As in the case of FIG. 1, the holes and cut-outs 8, 9, 10 are arranged between two holes 5 at the level of which are provided current lead-ins, not shown. Moreover, again as in the case of FIG. 1, the distribution of the holes and cut-outs 8, 9 and 10 is advantageously, as shown, selected so that there is obtained a temperature gradient in the direction of the arrow F, analogous to that described with regard to FIG. 1.

The cross-sections of the elements 1 according to the modifications of FIGS. 2 and 3 appear in FIGS. 20 and Again as in the case of FIGS. 2 and 3, there are re tained in the case of the embodiment of FIG. 11, at least certain of the principles which have prevailed in the constitution of that of FIG. 1. Here however, it was not sought to obtain a progressive temperature along the face F,. On the contrary, the cut-outs are practically symmetrical with respect to the median line XX. Although of generally different shape apparent from the Figure, there will be found again the'holes 2, the cut-outs 3a and 3b and the holes 5 of the preceding embodiments. There will also be found here too with a particular shape apparent from FIG. 11, parts 4 connecting the faces F and F,. F, and of the parts 3a as well as the thickness D of the parts 4 respect the above-mentioned criteria.

Advantageously, there are provided cut-outs 20 perpendicular to the face F,, as shown and of length H from 0.5 to 3cm. The cut-outs 20 are provided at the level of the parts 4.

The element thus constituted, such as shown in FIG. 11, offers good thermal and electrical insulation, good power-temperature ratio, good suspending possibilities, especially at the level of the holes 5, good elasticity, good resistance to forces of expansion and a large heating surface.

It will be realised that on the operation of an element ll, the expansions are absorbed by the parts 4 connecting the faces F, and F in the case of FIGS. 1 and 11, by the bridges P in the case of FIGS. 2 and 3.

Although in the case of the modifications of FIGS. 1 to 3 and 11, the ratio cavity/solid is in the neighbourhood of unity, this ratio is close to the lower limit of about 0.1 above-mentioned in the case of the advantageous embodiment of FIG. 12.

The element according to this embodiment comprises, as that of FIG. 11, a heating face F, of constant temperature in the useful part.

This element comprises two large cut-outs 20 of general direction parallel to F, forming a thickness D, of a heating surface F, (D, respects the above-mentioned criteria) and connected to the face F, by two cut-outs 21 of general direction perpendicular to F,, except a part 21a situated in the vicinity of the cut-outs 20 and oriented at about 45. The cut-outs 21 open on the face F On this face F, the element is hence formedof three parts 22, 23 and 24 comprising respectively cavities 25, 26 and 27 of which the first and third serve for the housing of the current lead-ins (not shown and very accesible due to this construction), the second having the purpose of lightening the whole of the structure.

In the feet 25a, 251).. 26a, 26b 27a, 27b (surrounding the corresponding cavities) there are provided holes 28 to 33 for the suspension of the element.

The part inclined at45 of the cut-outs 210 forms a screen and ensures the heat insulation of the heating surface F,.

In addition to the cut-outs 20 and 21, there are provided:

cut-outs 34 of general direction perpendicularto the face F, and situated in the mass of the element between the cut-outs 20 and the cavities 25, 26 and 27, these cut-outs 34, through their arrangement and situation which are apparent from the Figure, ensuring good conduction of the current and localising possible fissures caused by mechanical forces capable of being due to thermal thock and to centering of the constituant material;

cut-outs 35 provided in the vicinity of the ends of the elements, perpendicular and open on the face F,, these cut-outs playing the role of a heat shield and of releasing forces of expansion.

To enable the thickness of the heating part of heating path at the level of the face F, to be generally constant, there is provided between the cut-outs 20, a hole 36.

Lastly, in addition to longitudinal slots 7b similar to those provided in the embodiment of FIG. 1 and which thermally insulate the heating path, the face F, is cut-out along for example two cavities 37 and 38 which give it a T and a U profile shown in FIG. 13 by means of which a better behaviour in operation is achieved.

At the level of the face F there is advantageously provided a longitudinal cut-out 39 shown in FIG. 13.

The element according to this modification has not been also lightened as the preceding ones, the ratio cavity/solid being much less. On the other hand, it is of rapid and easy manufacture by reduction in the cutouts which are selected so that the propagation of fissures is avoided. It is well understood that it is by reason of the higher weight that the points of suspension are multiplied.

It is self-evident and emerges from the figures that, in the case of the embodiments corresponding to FIGS. 11 and 12, the current is obliged to pass through the face F,.

To establish ideas, it is indicated that in the case where the constituent material of the element is zirconia with 4 percent of lime the temperature of the face F, will be 1,200K when F, is at 2,200K.

This being the case, in a second embodiment, there is conferred on the element 1 the above-said nonhomogeneous structure along a direction substantially perpendicular to the facesF, and F, by making the said element comprise a part A of'low porosity, less than percent comprising especially the face F, which is intended to become conductive, this part with low porosity being combined with apart B of high porosity,

greater than percent comprising especially the face F the assembly being such that when the face F, is brought to a sufficient temperature to conduct electric current, the face F, remains sufficiently cool to be practically non-conducting. I

The part with low porosity can be constituted by stabilised zirconia similar to that constituting elements 1 in the embodiments of FIGS. 1 to 3.

The part of high porosity in which the pores play the role of cut-outs in the modifications of the preceding embodiments, can be constituted by the same material as the part of dense porosity, or again of a more economical material such as ZrO -AI O, eutectic, alumina or another refractory without chemical interaction with the zirconia.

In practice, the part of low-porosity which comprises the face F, will have a density of the order of 4.5 and the porous part comprising the face F will have a density of the order of 2.

To manufacture a refractory mass of given porosity necessary for the constitution of the part B comprising the face F recourse is had to conventional ceramic techniques, in particular moulding, pressing and sintering, (see for example Ceramic Fabrication Processes by W.D. Kingery, 1958, John Wiley).

Once part B has thus been constituted, the part A is attached to the part B, for example by projection, by having recourse to techniques using a plasma torch.

To establish ideas, it is indicated that to manufacture an element 1 which the distance between the faces F, and F, is 1 10 mm and of which the part comprising the face F, is formed of stabilized zirconia with a porosity of 6 percent and must be brought to a temperature of 22 K, recourse wil be had as regards the part comprising the face F and of which the temperature must not exceed 1,200 K at the level of the said face F to p0- rous materials of zirconia or other refractory oxide mixtures of 35 porosity. 1

This being so, there is shown in FIGS. 4, 5 and 6 three possible modifications for the constitution of the elements 1 constructed according to this second embodiment.

The modifications shown in FIGS. 4 and 6 are symmetrical with respect to a median plane in the direction of the length of the element.

There are distinguished in FIG. 4 holes 5 similar to those of FIGS. 1 to 3, and in FIG. 6 attached parts 13 also forming holes 5.

As in the preceding embodiment, the current lead-ins not shown are placed against the corresponding surfaces of the parts A, at the level of the holes 5.

Due to the fact of the symmetry of the modifications of FIGS. 4 and 6, there is no temperature gradient at the level of face F,.

To obtain such a temperature gradient, the part A can be arranged as is shown in FIG. 5, by conferring on it a diminishing cross-section in the direction of the arrow f, which indicates the direction of increasing temperatures.

As shown, the element shown in FIG. 5 comprises a hole T for suspension.

FIGS. 4a, 5a and 6a show cross-sections of the elements of FIGS. 4 to 6.

The heating elements according to the invention are distinguished by excellent behaviour in operation of long duration.

Thus an element corresponding to the modification shown in FIG. 1 and of which the face F, has constituted the heating wall of the furnace, has operated perfectly for 500 hours. At the end of this experiment, examination of the said element enabled no deterioration or change in its characteristics to be discerned.

The quantitiative values in the experiment were as follows:

temperature in the furnace chamber dimensions of the chamber dimensions of the elemtn 2,073K X 25 X I6 mm 220x ll0X30 mm thickness of the heating part (D,, FIG. I) 3.5 mm length of the heating part mm width of the heating part 30 mm applied voltage I67 V current intensity 8.7 A powder dissipated l.45 kW invention, it is possible to construct furnaces of anyshape and size.

In FIGS. 7 to 10, there are shown several possibilities for the constitution of the furnaces.

In FIG. 7, firstly there is shown an arch-shaped furnace with horizontal axis, constructed by means of elements 1 with a trapezoidal cross-section of the type of those shown in FIG.2. The various elements 1 rest on one another without it being necessary to resort to individual connections.

As is seen, the current lead-ins 6 are arranged as in the case of FIG. 1. There is shown the conductive wires connecting the current lead-ins 6 to the electrical current source.

The elements 1 used for the construction of the furnace with rectangular cross-section as shown in FIG. 8 are of the type of that of FIG. 1. Here again there is shown the current lead-in 6 and the wires 15. The three elements 1 constituting the sealing of the furnace are held in place by a mechanical suspension, preferably by a refractory steel fixing.

As seen in FIG. 9, it is possible to construct a furnace with a trapezoidal arch by resorting to elements 1 of the type of that of FIG. 3, spacing element 16, nonconductive of electricity and formed for example of zirconia or porous alumina, being then inserted between two elements 1. The maintenance in position of the elements 1 and 16 with respect to one another can be effected by means of a mechanical suspension (they can also be simply positioned on one another as in building games).

The three furnaces which have just been described have a heating cavity with a horizontal axis.

On, the other hand, FIG. 10 shows a modification with a vertical axis. In this case, there are used four elements 1 of the type of that shown in FIG. 1, spacing elements 17 being provided between the elements. The uniting of the elements 1 and 17 to one another is obtained by means of a shaping enclosure, containing and guiding the whole into good position.

To ensure the necessary pre-heating on starting of the constituant elements 1 of the above-said furnaces, recourse may be had in manner known in itself to a preheating element (not shown) introduced into the heating cavity to bring the faces F of the elements constituting the furnace to the minimum temperature at which the constituant material of these elements becomes conductive.

As a result of which, and whatever the embodiment adopted, there are thus provided heating elements of which the characteristics and operation emerge sufficiently from the foregoing for it to be unecessary to dwell further on this subject and which have, with respect to those pre-existing, numerous advantages, especially those:

of ensuring their own thermal insulation;

of enabling the construction of furnaces with a horizontal or a vertical axis and with a heating cavity having any desired heating gradient;

of having, due to the absence of mechanical strain, long life of several thousand hours continuously at 2,200K on the element;

of ensuring good operation even in the presence of cracks in the heating element,

of enabling the construction of furnaces of numerous geometries and of various capacities;

and of operation in an oxidising atmosphere. I

As is self-evident, and as emerges already from the foregoing, the invention is in no way limited to those of its types of application, nor to those embodiments of its various parts, which have been more especially indicated; it encompasses, on the contrary, all modifications.

We claim:

1. Heating element formed of refractory material resistant to oxidation, in the general shape of a parallelepiped, adapted to be heated by electric current supplied through electric contacts cladding said element, and having in a direction perpendicular to two of its opposite surfaces, a nonhomogeneous structure such that when one of said two surfaces is brought to a sufficiently high temperature by said electric current to become electrically conducting, the other remains at a sufficiently low temperature to be thermally and electrically practically non-conducting, said structure being adapted to withstand movements due to differences in expansion. 2. Heating element formed of refractory material resistant to oxidation, in the general shape of a parallelepiped, adapted to be heated by electric current supplied through electric contacts cladding said element, and having in a direction perpendicular to two of its opposite surfaces, a nonhomogeneous structure such that when one of said two surfaces is brought to a sufficiently high temperature by said electric current to become electrically conducting, the other remains at a sufficiently low temperature to be thermally and electrically practically non-conducting, said structure being adapted to withstand movements due to differences in expansion, constructed of at least partially stabilized zirconia, comprising holes and cut-outs formed in the mass of the element in a direction substantially perpendicular to that which unites said two opposite surfaces of which one is intended to be brought to a sufficient temperature to become electrically conducting, the ratio hollow/solid being selected in a range of about 0.1 to about 3.

3. Heating element according to claim 2, wherein the ratio hollow/solid is in the vicinity of unity.

4. Heating element according to claim 2, wherein the shape and the distribution of the holes and cut-outs are such that, on one hand the two opposite surfaces of which one must be brought to a sufficient temperature to be electrically conducting are connected to one another by portions of relatively slight cross-section and that, on the other hand, the electric current is obliged to pass through the surface of .which the'temperature is brought to a sufficient value for it to become conducting.

5. Heating element according to claim 4, wherein the distribution of the holes and cut-outs is selected so that the temperature reached by the surface through which the electric current passes is progressive along this surface.

6. Heating element according to claim 4, wherein the distribution of the holes and cut-outs is selected to that the temperature reached by the surface through which the electric current passes is uniform along this surface.

cut-outs of direction perpendicular to the heating surface and situated in the mass of the element between the cut-outs parallel to the heating surface, on the one hand, and the nonheating surface, on the other hand;

cut-outs perpendicular to the heating surface and opening on the latter in the neighborhood of the ends of the element;

and hollows opening on the nonheating surface.

8. Heating element formed of refractory material resistant to oxidation, in the general shape of a parallelepiped, adapted to be heated by electric current supplied through electric contacts cladding said element, and having in a direction perpendicular to two of its opposite surfaces, a non-homogeneous structure such that when one of said two surfaces is brought to a sufficiently high temperature by said electric current to become electrically conducting, the other remains at a sufficiently low temperature to be thermally and electrically practically non-conducting, said structure being adapted to withstand movements due to differences in expansion wherein along a direction substantially perpendicular to the assembly of the two opposite surfaces first part is formed of stabilized zirconia of density of the neighborhood of 4.5 and that the second part is formed of one of the materials comprising zirconia, eutectic zirconia A1 0 and alumina alone, the density of this second part being in the vicinity of 2.

10. Heating element according to claim 9, wherein the first part comprises a section decreasing in the direction of the length of the heating element.

11. Heating element according to claim 1, wherein the surface through the electric current passes is divided into lamellae, parallel to one another, by cut-outs perpendicular to the said surface in the direction of the length of the element.

12. Heating element according to claim 1, wherein the heating surface comprises hollows giving it a T and U structure, along a section perpendicular to the direction of the length.

13. Heating element according to claim 1, formed of a definite compound of the pyrochlore type based on zirconia, of the formula Zr T O where T is a metal, comprising holes and cut-outs formed in the mass of the element in a direction substantially perpendicular to that which unites said two opposite surfaces of which one is intended to be brought to a sufficient temperature to become electrically conducting, the ratio hollow/solid being selected in a range of about 0.1 to about 3.

14. Heating element according to claim 13, wherein said metal is a rare earth metal.

@33 I INITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 79 2, 2 33 Dated February 12 1974 v L l g 3 :i g I IEQEY Kr zys ztol DEMBINSKI Lucien DUPONT It is certified that error appears in the above-identified patent and that said Letters Patent are hereby cotrected as shown below:

In'the heading of the patent, the name of the Assignee should appear as follows:

Agence Nationale De Valorisation-De La Recherche (ANVAR) Paris, France Signed, and sealed this 15th day of October 1974.

(SEAL) Attest:

' MCCOY M. GIBSON JR. 0. MARSHALL DANN- Attesting Officer Commissioner of Patents a r I (5/69 "UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pa tent'No. 3,792,233 Dated February 12, 1974' a Y- Krz sztol DEMBINSKIH Lucien DUPONT Inventofls) If is certified that error appears in the above-ideritified "patent and that said Letters Patent arehereby eerrected as shown below:

Ifi the heading ef the patent, the name of the Assignee should appear as follows: 7

Agence Nationale De Valor-isation -De La Recherche (ANVAR) Paris, France SignedQand sealed this 15th day of October 1974.

(SEAL) Attest:

' MCCOY M. GIBSON JR.

Attesting Officer 0. MARSHALL DANN- Commissioner of Patents 

2. Heating element formed of refractory material resistant to oxidation, in the general shape of a parallelepiped, adapted to be heated by electric current supplied through electric contacts cladding said element, and having in a direction perpendicular to two of its opposite surfaces, a nonhomogeneous structure such that when one of said two surfaces is brought to a sufficiently high temperature by said electric current to become electrically conducting, the other remains at a sufficiently low temperature to be thermally and electrically practically non-conducting, said structure being adapted to withstand movements due to differences in expansion, constructed of at least partially stabilized zirconia, comprising holes and cut-outs formed in the mass of the element in a directIon substantially perpendicular to that which unites said two opposite surfaces of which one is intended to be brought to a sufficient temperature to become electrically conducting, the ratio hollow/solid being selected in a range of about 0.1 to about
 3. 3. Heating element according to claim 2, wherein the ratio hollow/solid is in the vicinity of unity.
 4. Heating element according to claim 2, wherein the shape and the distribution of the holes and cut-outs are such that, on one hand the two opposite surfaces of which one must be brought to a sufficient temperature to be electrically conducting are connected to one another by portions of relatively slight cross-section and that, on the other hand, the electric current is obliged to pass through the surface of which the temperature is brought to a sufficient value for it to become conducting.
 5. Heating element according to claim 4, wherein the distribution of the holes and cut-outs is selected so that the temperature reached by the surface through which the electric current passes is progressive along this surface.
 6. Heating element according to claim 4, wherein the distribution of the holes and cut-outs is selected to that the temperature reached by the surface through which the electric current passes is uniform along this surface.
 7. Heating element according to claim 1, wherein its ratio hollow/solid is in the vicinity of the lower limit of the range from 0.1 to 3 and comprising: cut-outs of general orientation parallel to the heating surface, these cut-outs being connected to the nonheating surface by cut-outs of the general direction perpendicular to the heating surface, certain of the perpendicular cut-outs comprising a portion inclined at about 45* and dividing the element into several parts; cut-outs of direction perpendicular to the heating surface and situated in the mass of the element between the cut-outs parallel to the heating surface, on the one hand, and the nonheating surface, on the other hand; cut-outs perpendicular to the heating surface and opening on the latter in the neighborhood of the ends of the element; and hollows opening on the nonheating surface.
 8. Heating element formed of refractory material resistant to oxidation, in the general shape of a parallelepiped, adapted to be heated by electric current supplied through electric contacts cladding said element, and having in a direction perpendicular to two of its opposite surfaces, a non-homogeneous structure such that when one of said two surfaces is brought to a sufficiently high temperature by said electric current to become electrically conducting, the other remains at a sufficiently low temperature to be thermally and electrically practically non-conducting, said structure being adapted to withstand movements due to differences in expansion wherein along a direction substantially perpendicular to the assembly of the two opposite surfaces of which one is intended to conduct the electric current, the said element comprises successively a first part with a slight porosity, less than 10 percent, comprising the surface intended to become electrically conducting, and a second part with a high porosity, greater than 30 percent.
 9. Heating element according to claim 8, wherein the first part is formed of stabilized zirconia of density of the neighborhood of 4.5 and that the second part is formed of one of the materials comprising zirconia, eutectic zirconia - Al2O3 and alumina alone, the density of this second part being in the vicinity of
 2. 10. Heating element according to claim 9, wherein the first part comprises a section decreasing in the direction of the length of the heating element.
 11. Heating element according to claim 1, wherein the surface through the electric current passes is divided into lamellae, parallel to one another, by cut-outs perpendicular to the said surface in the direction of the length of thE element.
 12. Heating element according to claim 1, wherein the heating surface comprises hollows giving it a T and U structure, along a section perpendicular to the direction of the length.
 13. Heating element according to claim 1, formed of a definite compound of the pyrochlore type based on zirconia, of the formula Zr2T2O5, where T is a metal, comprising holes and cut-outs formed in the mass of the element in a direction substantially perpendicular to that which unites said two opposite surfaces of which one is intended to be brought to a sufficient temperature to become electrically conducting, the ratio hollow/solid being selected in a range of about 0.1 to about
 3. 14. Heating element according to claim 13, wherein said metal is a rare earth metal. 